We are studying the mechanism of the cytotoxic agent, 6-thioguanine, on psoriasis and are doing this through blood kinetic studies and direct skin tissue studies. 6-thioguanine is a guanine analog which is incorporated into the DNA in the cell in place of the normal guanine component. This prevents the cell from replicating. Incorporation of this agent is much greater in young, rapidly growing cell populations than in older populations. The use of this drug in psoriasis makes sense, as psoriasis is a disease of rapidly proliferating keratinocytes, as well as proliferating lymphocytes with skin lesions.